1. Field of the Invention
The invention relates to a double-balanced mixer circuit comprising a first differential pair of first and second transistors whose emitters are connected to a common first emitter terminal, a second differential pair of third and fourth transistors whose emitters are connected to a common second emitter terminal, the bases of the first and the fourth transistor and the bases of the second and the third transistor being coupled to a common first and a common second base terminal respectively, between which base terminals a mixing signal may be applied, and a voltage-current converter having an input for receiving an input signal and having first and second outputs which are coupled to the first and the second emitter terminal respectively.
Such a mixer circuit may be used in, for example, radio and television receivers and in automatic mobile telephony receivers.
2. Description of the Related Art
Such a mixer circuit is in particular suitable for use in a one-chip integrated FM receiver as described in the Netherlands Patent Application No. 82.00.959, which receiver is commercially available as the integrated circuit TDA 7000. In this receiver the radio-frequency signal from the aerial is applied to the input of a voltage-current converter, which amplifies this aerial signal and applies the result in phase opposition to the common first and the common second emitter terminal of the first and the second differential pair. By means of an oscillator signal applied to the first and the second common base terminal the radio-frequency aerial signal is mixed down to form an I.F. signal.
An important parameter defining quality of a receiver is the dynamic range of the receiver. The dynamic range is the difference, expressed in dBs, between the minimum signal power required for intelligible reception and the maximum signal power for which reception which is not disturbed by intermodulation products is possible. As in the known receiver the aerial is connected virtually directly to the input of the mixer circuit, the dynamic range of the receiver depends almost entirely on the dynamic range of the mixer circuit. This is because noise and intermodulation products generated in the mixer circuit cannot subsequently be distinguished from the aerial signal. It has been found that in such circuits the quality of the mixer circuit up to very high frequencies is determined by the quality of the voltage-current converter.
In the known receiver the voltage-current converter comprises a differential amplifier which includes a pair of transistors which receive the radio-frequency signal as a single-ended drive, such collectors of the transistors being coupled to the first and the second common emitter terminals of the first and the second differential amplifiers of the mixer circuit. It has been found that the noise and, in particular, intermodulation behaviour of such a differential amplifier is insufficiently good to meet the stringent requirements imposed on the circuit when used in, for example, car radios. The intermodulation is caused by the non-linearity of the differential amplifier of the voltage-current converter. The linearity of the differential amplifier can be improved by arranging resistors in the emitter lines of the differential amplifier transistors, but this results in a deterioration of the noise behaviour.
In German Offlenlegungsschrift No. 31.40.417 it is proposed to dispense with the voltage-current converter differential amplifier. Instead of this amplifier a resistor is arranged in the common emitter lines of each of the differential pairs of the mixer circuit. The radio-frequency signal is applied across these resistors via coupling capacitors. However, this arrangement has the drawback that as a result of the direct signal path between the input of the oscillator signal and the aerial connection, interference originating from the oscillator is radiated to the aerial connection. Such interference is caused by the oscillator signal and harmonics of this signal.